The technique of transient electric dichroism is proving to be a very powerful method for determining structure and physical properties of DNA and DNA-protein complexes in solution. We are currently studying two systems of biological interest. There has been growing interest in a link between sequence dependent physical properties of DNA and cellular structures. Since most packaging of DNA in the nucleus involves bending, the persistence length of DNA is a directly pertinent physical characteristic. A significant dependence of DNA flexibility on dA-dT composition has been found. The energetics are such that DNA bending into structures as nucleosomes should show a strong dependence on dA-dT composition. In a second project, we are also studying the structure of DNA-DNA gyrase complexes under various. This enzyme catalyzes a supercoiling reaction of circular DNA that is necessary for efficient transcription in prokaryotic systems. The details of the mechanism are only vaguely known, mostly due to a lack of structural information about the interactions of DNA with the protein. The electric dichroism study will bridge this gap and provide valuable solution information about conformation and, more importantly, the conformational changes that take place.